The invention relates to a device for analysis of materials by means of radiation, including
a radiation source for producing the radiation,
a sample location for accommodating a sample of the material to be analyzed,
a position sensitive detection device for detecting the radiation emanating from the sample,
which detection device includes
an array of radiation sensitive detector elements,
an electronic read-out circuit which is connected to the detector array and includes charge amplifiers in a one-to-one relationship with the detector elements, the input of said charge amplifiers being connected to a respective one of the detector elements.
For the analysis of materials by means of radiation, notably X-rays, it is practically always necessary to measure the angle at which the radiation emanates from the sample relative to the direction of the incident beam. Measurement may concern a large angular range (of the order of magnitude of 180 degrees) or a smaller angular range when general knowledge has already been acquired as regards the angular distribution of the radiation in a given angular range. The latter is the case when, for example, the fine structure of an X-ray diffraction line is to be measured. In that case a position sensitive detection device can be used, that is, a detector which is provided with an array of adjacently situated (generally linear) radiation sensitive detector elements; the position of an element that is activated by radiation is then a measure of the angular direction in which the radiation emanates from the sample. In that case it is not necessary to use a detector that is adjustable in respect of angle; this offers considerable structural advantages.
United States patent U.S. Pat. No. 5,113,077 discloses a device for the detection of radiation in an apparatus for scanning an object by means of X-rays for computed tomography (xe2x80x9cX-ray CT scannerxe2x80x9d). The CT scanner described in the cited patent is provided with a position sensitive detection device for the detection of the X-rays emanating from an object to be examined. Said detection device includes an array of radiation sensitive detector elements in the form of a scintillator with photodiodes. An electronic read-out circuit is connected to each detector element, which read-out circuit includes charge amplifiers which are connected to the detector elements in a one-to-one relationship in such a manner that the input of a charge amplifier is connected each time to the output of one of the respective detector elements. Each of the charge amplifiers is constructed as a fed-back operational amplifier (OPAMP). A separate PNP transistor is connected to the output of the operational amplifier in such a manner that the output current of the combination formed by the OPAMP and the transistor is formed by the emitter current of the PNP transistor.
Analysis of materials by means of X-rays often utilizes comparatively soft X-rays, for example the Kxcex1 radiation of copper that has a wavelength of approximately 0.154 nm. The charge yield of a solid state detection element is comparatively small in the case of such comparatively soft X-rays. For example, per incident X-ray quantum an amount of charge corresponding to 2200 electron-hole pairs will be excited in a silicon detector consisting of a body of N material adjoining on the one side a PN junction and on the other side an N+ layer. In practice it is usually not possible to increase the X-ray yield of such a position sensitive detector by increasing the X-ray sensitive surface, because the dimensions of said surface determine the position resolution and hence are preferably kept as small as possible. Furthermore, the size of the surface of the detector element that is exposed to the X-rays defines the size of the take-off electrode. The combination of this take-off electrode and the semiconductor material on which it is arranged determines the value of the (stray) capacitance at the output of the detector element, that is, at the input of the subsequent charge amplifier. It is a generally known fact in such amplifier technology that the capacitance at the input of such an amplifier determines the noise contribution by said amplifier in the amplifier chain, meaning that the noise contribution will be higher as the input capacitance is higher.
Both of the above-mentioned effects (low charge yield per X-ray quantum and high input capacitance) lead to a comparatively low, that is, poor signal-to-noise ratio in the electronic read-out circuit of the detection device. In principle it is not impossible to enhance the signal-to-noise ratio by way of a longer measuring time; however, such a longer measuring time is undesirable because the comparatively expensive analysis apparatus would then take an undesirably long period of time for carrying out a measurement so that its efficiency would be very low.
In order to mitigate the described problem, it is an object of the invention to provide a device of the kind set forth in which the signal-to-noise ratio can be kept as high as possible during the further signal processing. To this end, the device in accordance with the invention is characterized in that the charge amplifiers (58) are constructed in the integrated bipolar technique, and that the electronic read-out circuit (48) includes signal processing circuits which are connected to the outputs of the charge amplifiers and are constructed in the digital technique.
The invention is based on the recognition of the fact that the stray input capacitance of the charge amplifier makes an important contribution to the noise. In order to minimize the noise contribution at that area, the integrated bipolar technique, causing a comparatively small amount of noise, is used for the charge amplifiers. The further signal processing should then take place at a speed which is high enough so as to avoid unnecessary limiting of the count rate and hence the throughput speed of the samples to be analyzed. This speed can be realized by way of signal processing circuits constructed in the digital technique. Moreover, circuits having a digital construction offer the additional advantage that they can be integrated with a comparatively small circuit surface area and that they can be constructed in such a manner that their power consumption is comparatively low. The requirements as regards small circuit surface area are even more severe since a sufficiently high count rate is required for the detector array; in order to achieve this, each detector element is provided with its own signal processing circuit, thus giving rise to a large number of processing circuits and hence a large surface area occupied by integrated circuits. In order to keep this surface sufficiently small nevertheless, use is made of the digital technique whose requirements as regards space are comparatively moderate.
It is to be noted that from the cited patent U.S. Pat. No. 5,113,077 it is known to provide the charge amplifier with a separate output transistor in the form of a PNP transistor (so a bipolar transistor). However, the cited patent does not provide any information concerning the technology used to construct the charge amplifiers themselves, that is, the circuits that are in direct contact with the stray input capacitance.
The digital signal processing circuits in a preferred embodiment are accommodated on the same substrate as the charge amplifiers. In order to realize a high position resolution and a large measuring range, preferably an as large as possible number of detector elements is used in a position sensitive detector array. This means that a one-to-one relationship between the charge amplifiers and the detector elements necessitates the use of an at least equally large number of connections. If each of the bipolar circuits and the circuits constructed in the digital technique were to have a respective substrate of its own, an equal number of bond pads would have to be provided for each connection on each substrate; as is known, bond pads occupy a large amount of space on the relevant integrated circuit. The above step offers the advantage that said bond pads can be dispensed with.
The digital signal processing circuits in a further embodiment of the invention are constructed by means of a BICMOS process in the form of the Current Mode Logic (CML) technique. If the digital signal processing circuits were constructed in the customary CMOS logic, upon transition from a first logic state to the complementary logic state a pulse-shaped peak current (also referred to as a reactive current) would occur from the transistors of the switched logic gates to the substrate on which they are provided. Because the charge amplifiers that are constructed in the bipolar technique are also mounted on the same substrate, such pulse-shaped leakage currents are liable to contribute to the noise of the charge amplifiers, thus degrading the signal-to-noise ratio thereof. When the above step is taken, this effect is avoided in that in the case of the BICMOS process the substrate has a resistance which is much higher than in the case of the customary CMOS process; consequently, the substrate currents are much smaller and penetrate less far into the substrate, so that the disturbing effect is less pronounced and reaches far less. Moreover, much smaller peak currents occur in BICMOS-CML circuits upon the transition from one logic state to the complementary logic state.
The assembly formed by the detector array and the electronic read-out circuits in a further embodiment of the invention is accommodated on a common substrate of a ceramic material. In order to achieve an optimum signal-to-noise ratio for the bipolar charge amplifiers, they are adjusted for a comparatively high quiescent current. The use of the CML technique also necessitates a current which is larger than that required in the case of the customary CMOS logic. Large currents involve a high dissipation of heat. When the circuits are accommodated on a ceramic substrate, for example aluminum nitride, a suitable distribution of heat is achieved, that is, a large heat sink surface area with its inherent suitable cooling properties.